diff --git a/README.md b/README.md
index fc0bac8cf..2369a0b60 100644
--- a/README.md
+++ b/README.md
@@ -72,6 +72,20 @@ Two locales, English default:
 
 Markdown overrides per locale live in `i18n/<locale>/docusaurus-plugin-content-pages/`. UI strings come from `i18n/<locale>/code.json` (generated by `npm run write-translations`).
 
+### Previewing both languages locally
+
+The Docusaurus dev server (`docusaurus start`) only serves **one language at a time**. `npm start` runs English; `npm run start:nl` runs Dutch. If you start the English server and click the language switcher, `/nl/` will render Docusaurus's internal "Page Not Found" — the route exists in the SPA but the locale isn't compiled in this dev process. The Dutch content is fine; the dev server just isn't serving it.
+
+To test the language switcher end-to-end (both locales on the same port, like production), build once and serve:
+
+```bash
+npm run preview
+# → http://localhost:3000/    (English)
+# → http://localhost:3000/nl/ (Dutch)
+```
+
+`npm run preview` = `docusaurus build && docusaurus serve`. After a build exists in `build/`, `npm run serve` alone reuses it without rebuilding. Trade-off: no hot-reload — re-run for every change.
+
 ## History
 
 This repository was migrated from the legacy Docusaurus v2.4.3 site in 2026 by extracting the new preset-based build from `design-system/sites/www/`.